Cyclodextrin is a cyclic oligosaccharide in which glucose is linked by α-1,4 bonds, and depending on the number of glucose units (n) that make up the ring, α-CD (n = 6), β-CD (n = 7), and γ-CD (n = 8) are known. CDs, which have many hydroxyl groups, dissolve well in water, but because they have a hydrophobic cavity made of hydrocarbons, they can encapsulate organic compounds that do not dissolve in water and make them soluble in water. By using this encapsulation function, for example, a dye-encapsulated CD and the reaction with a substrate can be used as an analytical reagent using the color change of the dye. Also, by utilizing the CD cavity like a nanosized molecular flask, it is possible to detect various metal ions, phosphate derivatives such as adenosine triphosphate (ATP), and sugars in water based on the absorption spectrum and fluorescence spectrum changes based on the molecular recognition reaction of the encapsulated color probe or fluorescence probe (Fig. 150.1a).